App Engine Standard J8 --> Stackdriver Monitoring Custom Metrics

pom.xml

<project>

  <modelVersion>4.0.0</modelVersion>
  <packaging>war</packaging>
  <version>1.0-SNAPSHOT</version>

  <groupId>com.test</groupId>
  <artifactId>root</artifactId>

  <dependencies>
    <dependency>
      <groupId>javax.servlet</groupId>
      <artifactId>javax.servlet-api</artifactId>
      <version>3.1.0</version>
      <type>jar</type>
      <scope>provided</scope>
    </dependency>

    <dependency>
    <groupId>com.google.cloud</groupId>
    <artifactId>google-cloud-monitoring</artifactId>
    <version>0.25.0-alpha</version>
    </dependency>

    <dependency>
        <groupId>com.google.auth</groupId>
        <artifactId>google-auth-library-appengine</artifactId>
        <version>0.8.0</version>
    </dependency>

    <dependency>
        <groupId>com.google.appengine</groupId>
        <artifactId>appengine-api-1.0-sdk</artifactId>
        <version>1.9.57</version>
    </dependency>

  </dependencies>

  <build>

    <outputDirectory>target/${project.artifactId}-${project.version}/WEB-INF/classes
    </outputDirectory>
    <plugins>

      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-jar-plugin</artifactId>
        <version>3.0.2</version>
        <dependencies>
          <dependency>
            <groupId>org.codehaus.plexus</groupId>
            <artifactId>plexus-io</artifactId>
            <version>2.0.5</version>
          </dependency>
        </dependencies>
      </plugin>

      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.5.1</version>
        <configuration>
          <source>1.8</source>
          <target>1.8</target>
        </configuration>
      </plugin>


      <plugin>
        <groupId>com.google.cloud.tools</groupId>
        <artifactId>appengine-maven-plugin</artifactId>
        <version>1.3.1</version>
        <configuration>
        </configuration>
      </plugin>

    </plugins>

  </build>

</project>

TestServlet.java

package com.test;

import com.google.appengine.api.appidentity.AppIdentityService;
import com.google.appengine.api.appidentity.AppIdentityServiceFactory;

import com.google.auth.appengine.AppEngineCredentials;

import com.google.api.gax.core.CredentialsProvider;
import com.google.api.gax.core.FixedCredentialsProvider;

import com.google.api.Metric;
import com.google.api.MonitoredResource;
import com.google.api.MetricDescriptor;

import com.google.cloud.monitoring.v3.MetricServiceClient;
import com.google.cloud.monitoring.v3.MetricServiceSettings;

import com.google.monitoring.v3.CreateTimeSeriesRequest;
import com.google.monitoring.v3.MetricDescriptorName;
import com.google.monitoring.v3.Point;
import com.google.monitoring.v3.ProjectName;
import com.google.monitoring.v3.TimeInterval;
import com.google.monitoring.v3.TimeSeries;
import com.google.monitoring.v3.TypedValue;

import com.google.protobuf.util.Timestamps;

import java.io.IOException;

import java.text.NumberFormat;

import java.util.Arrays;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Locale;
import java.util.logging.Logger;
import java.util.Map;

import javax.servlet.http.*;

public class TestServlet extends HttpServlet {

    private String projectId = "[[YOUR-PROJECT-ID]]";
    private static final Logger log = Logger.getLogger(TestServlet.class.getName());
	
	public void doGet(HttpServletRequest req, HttpServletResponse resp)	throws IOException {
		try {
            resp.setContentType("text/plain");
            resp.getWriter().println("Hello, Henry!\n");
        
            AppIdentityService appIdentityService = AppIdentityServiceFactory.getAppIdentityService();

            List<String> scopes = Arrays.asList("https://www.googleapis.com/auth/monitoring");

            appIdentityService
                .getAccessToken(scopes)
                .getAccessToken();

            AppEngineCredentials credentials = AppEngineCredentials
                .newBuilder()
                .setAppIdentityService(appIdentityService)
                .setScopes(scopes)
                .build();

            CredentialsProvider credentialsProvider = FixedCredentialsProvider
                .create(credentials);

            MetricServiceSettings metricServiceSettings =
                MetricServiceSettings.newBuilder()
                    .setCredentialsProvider(credentialsProvider)
                    .build();
            MetricServiceClient metricServiceClient = MetricServiceClient
                .create(metricServiceSettings);        
            
            Runtime rt = Runtime.getRuntime();
            long free = rt.freeMemory();
            resp.getWriter().println("Free: " + NumberFormat.getNumberInstance(Locale.US).format(free));
            placeHolder2(metricServiceClient, resp, free);

            TimeInterval interval = TimeInterval.newBuilder()
                .setEndTime(Timestamps.fromMillis(System.currentTimeMillis()))
                .build();
            TypedValue value = TypedValue.newBuilder()
                .setInt64Value(metricPoint)
                .build();
            Point point = Point.newBuilder()
                .setInterval(interval)
                .setValue(value)
                .build();

            List<Point> pointList = new ArrayList<>();
            pointList.add(point);

            ProjectName name = ProjectName.create(projectId);

            HashMap<String,String> metricLabels = new HashMap<String,String>();
            metricLabels.put("memory", "free");
            metricLabels.put("test", "salcode");

            Metric metric = Metric.newBuilder()
                .setType("custom.googleapis.com/instance/memory")
                .putAllLabels(metricLabels)
                .build();

            Map<String, String> resourceLabels = new HashMap<String, String>();
            resourceLabels.put("project_id", projectId);

            MonitoredResource resource = MonitoredResource.newBuilder()
                .setType("global")
                .putAllLabels(resourceLabels)
                .build();

            TimeSeries timeSeries = TimeSeries.newBuilder()
                .setMetric(metric)
                .setResource(resource)
                .addAllPoints(pointList)
                .build();
            List<TimeSeries> timeSeriesList = new ArrayList<>();
            timeSeriesList.add(timeSeries);

            CreateTimeSeriesRequest request = CreateTimeSeriesRequest.newBuilder()
                .setNameWithProjectName(name)
                .addAllTimeSeries(timeSeriesList)
                .build();

            metricServiceClient.createTimeSeries(request);

            metricServiceClient.close();
        
    } catch (Exception ex) {
		  resp.getWriter().println("Exception " + ex);
		}

}